The present invention relates to a magnetic recording medium having two magnetic layers, more particularly a magnetic recording medium having an upper magnetic layer of a higher coercive force and a lower magnetic layer of a lower coercive force, which has improved sensitivity and frequency characteristics at low and high frequency bands with less noise.
It is known that a magnetic recording medium having two magnetic layers a lower layer of a lower coercive force and an upper layer of a higher coercive force shows improved sensitivity and frequency characteristics at low and high frequency bands, and further that when magnetic metallic particles having a large maximum magnetization are used as the magnetic particles for the magnetic layer, the sensitivity and frequency characteristics are further improved. Such a magnetic recording medium is usually produced by first applying a magnetic paint composition containing magnetic metallic particles of a comparatively lower coercive force onto a base film, such as polyester film, and drying to form a lower magnetic layer, and secondly applying a magnetic paint composition containing magnetic metallic particles of a comparatively higher coercive force onto the lower magnetic layer and then drying to form an upper magnetic layer.
However, the conventional magnetic recording medium having two magnetic layers obtained by using magnetic metallic particles has a difference in the coercive forces between the upper and lower magnetic layers of less than 100 oersteds, and such a magnetic recording medium shows insufficient improvement in the sensitivity and frequency characteristics at low and high frequency bands and still shows noise.
The present inventors have intensively studied so as to improve the magnetic recording medium having two magnetic layers concentrating on the difference of coercive force between the upper and lower magnetic layers. As a result, it has been found that a desired magnetic recording medium of two magnetic layers having excellently improved sensitivity and frequency characteristics at low and high frequency bands with less noise can be obtained by forming a lower magnetic layer having a coercive force of at least 100 oersteds lower than that of the upper magnetic layer, making the ratio of thickness between the upper magnetic layer and the lower magnetic layer (i.e. the thickness of the upper magnetic layer/the thickness of the lower magnetic layer) in the range of 1/1 to 1/9, and further making the total coercive force of the upper and lower magnetic layers 1,000 oersteds or more.